推 yyiitt1003:感謝 11/13 01:35
※ 引述《yyiitt1003 (懷念補習班的日子)》之銘言：
: 以下問題,實在想不出來思考盲點,請各位高手幫忙解惑了~
: Running at their respective constant rates, macgine X takes 2 days
: longer to produce w widgets than machine Y.At these rates,if the
: machines together produce 5/4w widgets in 3 days,how many days would
: it take machine X alone to produce 2w widgets?
: Ans.12
這題w根本是擾亂你的 把w看成全部的進度,當成100%即可
量= T*S, S=量/T假設X花d+2天完成w，Y花d天完成w
X的產率w/(d+2), Y的產率w/d, 一起工作3天產生出5w/4,
[ w/(d+2) + w/d ]=5w/4除以3, 12/d+12/(d+2)=5d(d+2)
得~ 12d+24+12d=5d^2+10d , 5d^2-14d-24=0
(5d+6)(d-4)=0, d= -6/5,4~ X產W需要4+2=6天
題目問2W,所以2*6=12 天~ 選(E)
: A certain roller coaster has 3 cars,and a passenger is equally likely
: to ride in any 1 of the 3 cars each time that passenger rides the roller
: coaster. IF a certain passenger is to ride the roller coaster 3 times,
: what is the probability that the passenger will ride in each of the 3 cars?
: Ans. 2/9
: There are 8 books on a shelf,of which 2 are paperbacks
: and 6 are hardbacks.How many possible selections of 4 books from this
: shelf include at least one paperback?
: Ans. 50
: 做越多題,越來越沒信心了..
選3輛，可以重複選，所以分母就是3*3*3，分子(3*2*1)是這樣的，
第一次可以隨便選，第二次是剩下的2個選1個，最後一次就是最後剩下的那輛，
3*2*1/3^3= 2/9 選(C)
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